Study: BP Oil Spill Led to Biggest Gulf Dolphin Die-Off Recorded

Dolphin Deaths BP Oil Spill
A dolphin that was stranded and died off the Louisiana coast following the Deepwater Horizon oil spill is photographed for research in the summer of 2012. Louisiana Department of Wildlife and Fisheries

A study released by the federal government Wednesday adds certainty to the conclusion that the 2010 BP oil spill led to an ongoing spike in bottlenose dolphin deaths in the Gulf of Mexico.

Lesions discovered on the lungs and adrenal glands of dead dolphins found within the footprint of the Deepwater Horizon spill are clear indications of exposure to oil products, according to National Oceanic and Atmospheric Administration (NOAA) researchers involved in the study.

"No feasible alternative causes remain," Stephanie Venn-Watson, a veterinary epidemiologist who led the study, told reporters Wednesday. The lesions found by the researchers indicate that many of the dolphins had bacterial pneumonia and adrenal disorders that likely caused their deaths.

BP Oil Spill Dolphin Deaths
"These dolphins had some of the most severe lung lesions I have seen," Kathleen Colegrove, the study's lead veterinary pathologist, said. Louisiana Department of Wildlife and Fisheries

The adrenal gland in dolphins produces hormones that regulate vital functions like blood pressure and metabolism. (It performs the same functions in the human body.) When something is amiss in the adrenal gland, the animals are at high risk of death. "Adrenal crisis," an event caused by adrenal insufficiency, can produce a constellation of life-threatening health issues in a dolphin.

Nearly half the dead dolphins recovered between 2010 and 2012 from Barataria Bay, Louisiana, had a thin adrenal gland cortex, indicating insufficient adrenal function, and died "without another clear explanation," according to the research. Barataria Bay was one of the areas along the Louisiana coast most heavily oiled from the spill.

By comparison, only 7 percent of dead dolphins from the "control group," which did not die within the oil spill footprint, showed this adrenal abnormality.

"Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives, and when those stressors occur, they are more likely to die," Venn-Watson said.

Bacterial pneumonia was also rampant among the dolphin carcasses surveyed. Lesions from the infection were present in over 20 percent of the dead dolphins. Only 2 percent of dead dolphins in the control group had these lesions.

"These dolphins had some of the most severe lung lesions I have seen in the over 13 years that I have been examining dead dolphin tissues from throughout the United States," said Kathleen Colegrove, the study's lead veterinary pathologist, based at the University of Illinois, in a statement.

Dolphins take deep breaths of air right at the water's surface, where oil fumes would be most concentrated, and hold that air in their lungs for long periods of time while they dive. "Dolphins were swimming through oil" in the aftermath of the spill, Venn-Watson says, making them extremely susceptible to developing the condition.

Dolphins Swim Through Oil from BP Spill
Dolphins swim through oil in the Gulf of Mexico during the Deepwater Horizon oil spill in April 2010. NOAA

The spike in deaths, referred to in scientific literature as an Unusual Mortality Event (UME), began in 2010. Now, five years after the spill, the UME—the largest ever recorded in the northern Gulf of Mexico—is still ongoing. At various points since the UME began, the rate of dolphin deaths has been recorded at four to five times greater than what is considered normal for the area.

Wednesday's paper builds on a series of previous peer-reviewed studies that have pointed to a similar conclusion. In February, NOAA researchers found that "dolphin deaths in Louisiana for 2010 and 2011 were the highest ever recorded for that state," said Teri Rowles, the head of NOAA's Marine Mammal Health and Stranding Response Program, at the time.

"These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf's dolphin population," Rowles said in a statement Wednesday. "This [latest] study carries those findings significantly forward."